Magnesium-zinc paint pigments



United States Patent 2,877,126 MAGNESIUM-ZINC PAINT PIGMENTS LawrenceWhitby, Bay City, Mich., assignor to The Dow Chemical Company, Midland,Mich., a corporation of Delaware No Drawing. Application November 5,1956 Serial No. 620,203

3 Claims. (Cl. 106290) incorporate particulate zinc metal inanti-corrosive priming paints for use on iron and steel. It is alsoknown to produce a rust-inhibitive aluminum pigmented paint by treatingaluminum particles with a fatty acid, flaking the particles, mixing thethus-treated particles in a strontium chromate paint paste, anddispersing the mixture in a vehicle.

The dispersed zinc metal alone ofiers but limited protection againstcorrosion to iron and steel and no observed protection to aluminum ormagnesium surfaces. The fatty acid treated aluminum flake results in theflakes being insulated by the fatty' acid and, as a result, forms adiscontinuous layer or coating. Such coating, although enhancing theappearance of a surface and offering some degree of protection of aphysical nature, afiords but small protection against corrosion.

There is a desideratum in the art for a zinc chromate or alkaline earthchromate-pigmented coating composition that contains metallicparticulate bodies which, when applied to metal surfaces, e. g.,articles of iron, aluminum, magnesium, or their alloys, afiordsprotection :against corrosion and enhances the appearance of such metalsand alloys to a greater extent than do coating compositions now known.There is an especial need for such a coating composition which isuniversally applicable as a primer or base coating composition for ironand aluminum-base alloys and which provides an appreciable degree ofprotection to uncoated areas which are adjacent to areas coated by suchcomposition. Scuffs and scratches in the coated surface as wellascorners and edges not fully coated during the coating operation arethereby provided with some protection.

Accordingly, the principal object of the invention is to provide apigment containing particulate metallic bodies useful in the preparationof pigmented coating compositions for their protection and for enhancingthe appearance of metal articles, e. g., those of iron, aluminum, andmagnesium and their alloys.

Another object of the invention is to provide a pigment especiallysuitable for the preparation of a primer or base coating for metalarticles.

The invention then comprises a novel pigment for making pigmentedcoating compositions containing particulate bodies of magnesium-zincalloy and a chromate selected from the chromates of zinc and alkalineearth metals, e. g., strontium, the method of preparing such a'pigment,and the pigmented coating composition containing such pigment.

According to the invention, substantially pure magnesium and zincmetals, in a ratio of between 70 and 85 percent zinc to between 30 and15 percent magnesium 2,877,126 Patented Mar. 10, 19.59

by weight are fused together preferably in the presence of a suitableflux, into a substantially homogeneous mass thereby forming a brittlealloy of magnesium and zinc. Among the alloy compositions which may beformed by thus fusing magnesium and zinc are the brittle intermetalliccompounds of MgZn and MgZn If desired, the temperature may be held atslightly above the fusion temperature for a short time, say .25 to .75hour to insure complete fusion. The thus-formed alloy is cast into ablock or other shape by pouring it into a mold and cooling it. Thethus-molded block which is of a brittle nature is broken into bits by ahammer, pestle or other means and pulverized, as by grinding in a ballmill, to a fineness sufficient to permit at least 98 percent to passthrough a No. 325 standard mesh screen (United States standard sieveseries).

The thus-pulverized magnesium-zinc alloy is admixed with finelyparticulated, e. g., pigment grade, zinc chromate or an alkaline earthchromate and a conventional coating vehicle used for chromate pigments.The pulverized magnesium-zinc alloy may be mixed with the chromate andthen admixed with the vehicle or they may be added directly to thecoating vehicle, which either already contains at least one of the abovechromates, or to a suitable vehicle alone and subsequently adding thechromate to the vehicle containing the pulverized magnesium-zinc alloy.

The ratio of the pulverized magnesium-zinc alloy to the zinc or alkalineearth chromate in the mixture so ob: tained is between :20 and 20:80 byvolume. The preferable range is about 50:50 on a volume basis.

By a conventional coating vehicle is meant any organic liquid filmformer commonly used in the 'paint and varnish industry in paint,varnish, lacquer, primer, and enamel formulations, e. g., drying oilssuch as linseed oil or a linseed oil-modified alkyd resin plus avolatile thinner as required such as a mineral spirit or xylene.- Aliquid vehicle, containing the oil-modified resin, is referred to in theart as a bodied resin-oil vehicle. The vehicle suitable for use in theinvention may also contain such additaments, in amounts employed in theart, as an anti-skinning agent, e. g., dipentene; a gelling agent, e.g., 1 to 10 percent aluminum stearate gel in xylene; an extender such asasbestine; and a drier such as" a naphthenate or oleate of manganese,lead, or cobalt. Other pigments, e. g., iron oxides or organic dyes maybe present or added to the vehicle. The pulverized 'n'ia'gnesium-zincalloy, together with the chromate and vehicle containing otherfunctional ingredients as desired and as described above, may beintermixed in accordance with known-practice as in a ball mill or rollermill. f

The coating compositions formulated according tothe invention possessexcellent can-storage stability and may be readily applied to metalsurfaces by the usual methods of application, e. g., brushing orspraying. They possess excellent drying properties.

The preferred chromate to be employed in practicing the invention iszinc chromate. The coating produced when zinc chromate is applied as thechromate of the invention is of an olive drab color which is highlystable to light and atmospheric conditions.

The following examples illustrate variations in the formulation of thepigmented coating of the invention. The effectiveness of the coatinginprotecting panels of steel, and aluminum-base and magnesium-base alloysis shown by application of the coating of the inventionto such panelsand subjecting the thus-coated panels to c0rrosion tests which areexplained hereinafter. Formulations containing only one of either themagnesium-zinc alloy powder or zinc chromate were also run as blanks 3for comparative purposes. ,They show the high suseep'e tibility toattack of narrow uncoated areas on the panels of mild steel andaluminum-base alloy when the adjacent area of the panels is notprotected by the pigmented coating of the invention. The blanks alsoshow the higher degree To a third batch of the same vehicle compositionwere admixed 280 grams of zinc chromate to prepare coating composition Cused to coat the sample panels designated blanks b, d, h, j, and l ofTable III.

of corrosion suffered by panels of magnesium-base alloy 5 To prepare thecoating compositions used to coat the as compared to panels coated bythe novel coating companels of Examples 1 to 18 of the invention, as setout position of the invention. in Table III, the following ratios ofcompositions A, B, For use in the blanks for comparison purposes and inand C, set out in Table II were used. Coating composithe examples of theinvention, a composition, comprising tions A, B, and C contained equalvolumes of the piga vehicle and non-pigment solid ingredients, wasprepared ments in equal weights of vehicle composition. Since by mixingtogether the following ingredients in the prothe pigments weresubstantially uniformly dispersed in portions shown in Table I. thevehicle, proportionate weights of the Vehicle con- TABLE I tained thesame proportionate volume of the pigment.

Ingredient Grams TABLE H Asbestlnedargelyfibrous magnesiumsilicate) 96.00 Percent Percent Batch A Batch B Batch 0 10 percent aluminum stearatein xylene 80. 00 Mg-Zinc ZnCrOi by by by Examples in Medium oil-lengthlinseed oil-modified alkyd resin, 60% by by by Weight 1 Weight Weightwhich used weight lnxylene 548.00 Volume Volume Phenol-formaldehyderesin, 50% by we ht an aliphatic 20 33rd: .II 75 1 1, 7 and 13. Xylene50 50 2 2. 8, and 14. Dipentene 25 75 3 3, 9, and 15. Lead naphthenate75 25 l 4. 10, and 16. Cobalt naphthenate 50 50 2 5. 11. and 17. 25 2575 3 6, 12, and l8. A number of batches of the vehicle composition ac-PM? cording to 'flable Ilwere prepared to be used in the mangsgcgg=g9gramsoggagcllilitltlle velklliclle accordiing t o I lalt))l1e ereinater exp i a c i gramso a c eve lceaccor mg 0 a e The vehicle compositionof Table I is illustrative of to a g l e% grams of mm chromate theVehicle according but one of a wide selection of vehicles known asresinbodied vehicles which are acceptable in the coating art The I' akFOfHPOSltlOl'lS and examples prefor use in zinc chromate-base paints.Any vehicle which pared as described hereihabove used to coat metal isacceptable for use with zinc chromate-base paints is Panels P FP andcoated as explained below. satisfactory for the practice of theinvention. Metallic Panels having the dimensions of X X Themagnesium-zinc binary alloy for use with zinc composfid mild alloySibel, aluminum-base chromate in the above vehicle composition inaccordance loyfiesighated Aluminum Association 2024 and with theinvention Was prepared as follows: h l alloy AZ31 respectiybly, werecleaned,

Two batches of magnesium were weighed and melted P rinsed, and driedaccording to known Practicesinian induction furnace in the presence of aflux compris- The composition of the mild Steel is Set out broadly in i55 grams c 34 grams cl 9 grams Back, 2 40 Metals Handbook of the Societyfor Metals (1948), page grams c p and 5 grams 1 preheated Zinc metal534. The composition of the aluminum-base alloy is was then added toeach of the batches of molten magthat designated ASTM The AZ31 nesium togive the stoichiometric combining Weights of nesium-base alloy has anominal Composition of 3 Peffient 65.38 grams of zinc to 24.32 grams ofmagnesium to aluminum, 1 Percent Zinc, about Percent manganese, formbatch X and 130.76 grams of zinc to 24.32 grams and the balancemagheshlm- The Panels were evenly of magnesium to form batch Y,respectively. These coated y brush application of the coatingCompositions stoichiometric amounts gave percentage compositions of P pas described above- 12.9% zinc and 27.1% magnesium in batchX and 84.3%The percent composition of the paints used to coat zin d 157% magnesiumi b t h Y Th ll the panels is set out in Table III (the percentmagnesium om o iti a tho shown i T bl 111 and zinc in the pulverizedalloy is by weight). The per- Th temperature of th lt f b h X d Y centof the magnesium-zinc alloy and the zinc chromate was raised to 1400 F.and superheated at this temperain the mating compositions are Set 01113y Volumeture for 30 minutes. The superheated batches were The coated paels Were tested y subjecting them to then poured into preheated moldsrespectively, and therethe action of a corrosive atmosphere 0f either aSalt Spray after cooled. Each of the brittle alloys of magnesium g as inA B-11754T test, the humid air and zinc thus formed was broken into bitsand then septest explained belOW- The Salt p y test, as pp ratel placedi a pebble mill a d pulverized the in blank panels and panels coatedaccording to the invento form alloy batches, the particle size of whichwas re o comprises subjecting the panels to a fine mist of ducedsufiiciently to permit 98 percent of each of the salt solution made upof about 20 parts of sodium chlothus-formed pulverulent alloys to passthrough a No. ride dissolved in 80 Parts Of Water y Weight, in a S11b-325 standard si stantially corrosion-proof box maintained at 95 F. TheTo prepare the coating compositions used in the blanks humid air testcomprises exposing coated test panels to and the examples set out inTable III, zinc chromate and air of 95 percent or more relative humidityat 95 F. portions of the batches of the magnesium-zinc alloys X r e in ig coatings p p in the oXamplss, the and Y, prepared as above described,were admixed um panels upon which the pigmented coating had been apderan agitator to successive batches of the vehicle complied were, prior tobeing subjected to the test, scratched position prepared according tothe formulation of Table I. diagonally from corner to corner with aknife which was To a first batch of the vehicle composition of Table Imade to penetrate entirely through the coating to the were admixed 428grams of binary alloy X to prepare metal. The corrosive atmosphereprovided for the panels coating composition A used to coat the samplepanels and the time of exposure are set out in Table III. designated a,e, and i inTable III. 'The protection afiorded by the coating formed byap- To a second batch of the same vehicle composition plication of thenovel pigment coating of the invention were admixed 336 grams of binaryalloy Y to prepare and coatings formed on blanks for comparison wereasthe coating composition B used to coat sample panels certain ed bythree tests: (a) percent corrosion in and designated blanks c, g, and kin Table III. along scratch, (12) width of corrosion spreading fromcorroded and calculating what percent the corroded distance was of theentire length of the scratch. The pur pose of this test was to show theextent of protection aiforded to uncoated areas, viz, scratches, whenadjacent to a coated area.

The second test, the width of corrosion spreading from 1 a scratch,consisted of measuring at right angles to the scratch the lineardistance from the scratch to which the coating was loosened from themetal panel; the average of such distances in millimeters measured atseveral places along the scratches was recorded. The purpose of thistest was to show the effect of the corrosion on the bond between thecoating composition and the metal.

The third test, percent of painted surface of panels corroded, took intoconsideration the area exposed, exclusive of the portions of the areataken up by the scratches. This test consisted essentially of measuringthe unscratched area of the coated surface, which was corroded, andcalculating the percentage that the thus-measured corroded area was ofthe original coated surface, exclusive of the scratches. The purpose ofthis test was to show the protection afforded to metal surfaces whichare covered by the novel coating composition of the invention.

Table III sets forth: the type of panels coated, the binary alloys usedin the coating composition, the volume ratio of the binary alloy to zincchromate in the coating, and the test results.

are adjacent to areas coated by the novel coating composition of theinventor. The uncoated areas on the panels are illustrated by thescratches which penetrated the coating to expose the bare metal. Thisprotection has wide application in protecting such alloys after thecoating has been scufied, marred or scratched in usage as well asafiording protection to crevices and edges which are not readilyaccessible or are easily overlooked when the coating is applied.

The results of the tests alsoshow that the protection afforded by thenovel pigmented coating of the invention is at least as good as straightzinc chromate-base paint in the protection of aluminum, slightlysuperior in the protection of steel, and definitely superior in itsprotection of magnesium in a highly corrosive atmosphere. While theexamples of the invention have employed zinc chromate, the alkalineearth chromates, e. g., chromates of barium, strontium and calcium, maybe used in the same volume ratios to the magnesium-zinc binary alloy aswas the zinc chromate. For example, when SrCrO is substituted for thezinc chromate in the examples, the results obtained compare favorablywith those obtained with Zinc chromate.

Having described the invention, what is claimed and desired to beprotected by Letters Patent is:

l. A pigment for pigmenting coating compositions, said pigmentconsisting of an admixture of (a) between 80 and 20 percent by volume ofa particulate binary alloy consisting of magnesium in an amount betweenand 15 weight percent and zinc in an amount between and weight percentand (b) between 20 and 80 percent by volume of a chromate selected fromthe class consisting of chromates of zinc and alkaline earth metals.

TABLE 1H Test results of panels protected by coatings of MgZn and ZnCrOVolume Percent Designation of Mg-Zn Alloy Test Results and ZnCrOr AlloyBatch in Com- Percent Width of Percent position Atmosphere CorrosionCorrosion Coated Panel Coated Test N0. Mg-Zinc ZnCrOi and Time in andSpread Surface Alloy Exposed along From Showing Scratch Scratch,Corrosion mm.

Blank 11..... Batch X-- 75 50 Mild A Steel do Blank dc Blank c. Batch X.

Aluminum base Alloy N0.

Magnesium base Alloys AZBI Ex. 18 Blank I"--.

Salt Spray H e: w OHOOQEHOO cpoooocooo oocooencoco 5 Batch X=27.1% Mg,72.9% Zn, z MgZn; Batch Y=15.7% Mg, 84.3% ZngMgZng.

The results of the tests set forth in Table III show the protectionprovided by the novel coating composition to uncoated areas on steel andaluminum-base alloys which 75 and 20 percent by volume of anintermetallic binary 2. A pigment for pigmenting coating compositions,said pigment consisting of an admixture of (a) between 80 compoundcontaining--stdichiometric quantities of magnesium and zinc-metals and bbetween-20 and 80 percent by volume of a chromate selected from theclass consisting of the chromates of zinc and alkaline earth metals,said binary being within the range of 70 to 85% of zinc and 30 to 15% ofmagnesium.

3. A pigment according to claim 1 wherein the chromate is zinc chromate.

References Cited'in the file of this patent UNITED STATES PATENTS1,468,407 Schrotter Sept. 18, 1923 8 Bullitt Mar. 16, George Oct. 5,Sloan Nov. 11,

FOREIGN PATENTS Great Britain Dec. 22,

Great Britain Mar. 10,

1. A PIGMENT FOR PIGMENTING COATING COMPOSITION, SAID PIGMENT CONSISTINGOF AN ADMIXTURE OF (A) BETWEEN 80 AND 20 PERCENT BY VOLUME OF APARTICULATE BINARY ALLOY CONSISTING OF MAGNESIUM IN AN AMOUNT BETWEEN 30AND 15 WEIGHT PERCENT AND ZINC IN AN AMOUNT BETWEEN 70 AND 85 WEIGHTPERCENT AND (B) BETWEEN 20 AND 80 PERCENT BY VOLUME OF A CHROMATESELECTED FROM THE CLASS CONSISTING OF CHROMATES OF ZINC AND ALKALINEEARTH METALS.